1. Field of the Invention
The present invention relates to a motion detecting device for detecting the motion of a subject that is reproduced from field to field as a motion picture is played back.
2. Description of the Prior Art
In interlaced scanning, a picture is shot by performing image sensing alternately in the pixels in even-numbered lines and in the pixels in odd-numbered lines for each field of the picture. Thus, when such a picture shot by interlaced scanning is played back, it is necessary to interpolate the image data for those pixels to which no image data is allotted in a given field. One method of interpolating image data for pixels to which no image data is allotted is spatial interpolation whereby interpolation is achieved using the image data of a plurality of pixels nearby, and another is temporal interpolation whereby interpolation is achieved using the image data of identical pixels among a plurality of fields before and after the one currently being reproduced.
Whether to rely on spatial or temporal interpolation to interpolate necessary image data is decided by detecting the motion of a subject in the picture. Specifically, spatial interpolation is used to interpolate image data for pixels that are sensing a subject in motion, and temporal interpolation is used to interpolate image data for pixels that are sensing a motionless subject. In this way, by switching interpolation methods according to the state of motion of the subject being sensed by individual pixels, it is possible to faithfully reproduce the sensed subject in each field of the picture being played back.
Conventionally, such detection of motion is achieved by calculating differences of the image data of identical pixels among even-numbered and odd-numbered fields, and then comparing those differences with a predetermined threshold value. Specifically, to detect the motion of a subject that is being sensed by the pixels for which image data is going to be interpolated, the differences between the image data obtained from those pixels one field before and the image data obtained from the same pixels three fields before are compared with a predetermined threshold value, and, if the differences are greater than the threshold value, the subject being sensed by the pixels in question is recognized to be in motion.
In this way, by comparing the field-to-field differences of the image data of identical pixels with a predetermined threshold value, whether the subject being sensed by the pixels for which image data is going to be interpolated is in motion or not is judged. However, as long as such a threshold level is kept constant, for example, in a case where motion was present up to the field immediately previous to the one currently being reproduced but no motion is present any more in the current field, the motion that had been recognized just up to the previous field leads to an erroneous judgment that the motion is still present in the current field. In this case, although the image data of adjacent pixels now have nearly equal values and therefore spatial interpolation is appropriate, in fact temporal interpolation is performed. This makes faithful reproduction of the real image impossible, and sometimes causes flickering or the like while a motion picture is being played back.